The present invention relates to a sleeve, and in a particular to an arrangement for securing a screw, such as a tuning screw, in the sleeve.
In many applications it is of interest to secure a screw. One such application is tunable low-pass and band-pass filters for use in the radio frequency range.
Such filters are designed to allow electromagnetic frequencies within a predetermined range pass through the filter and to block frequencies outside that range. Furthermore, in Microwave Filters, Impedance-Matching Networks, and Coupling Structures, by G. Mattaei, L. Young, and E. M. T. Jones (Artech House Booksxe2x80x94Dedham, Mass., Copyright, 1980), the design of such filters is described in detail.
Further, low-pass and band-pass filters for use in the radio frequency range typically comprises a filter housing having a number of resonator members located inside it. The filter is tuned to have desired characteristics by means of tuning screws located above the resonators, which screws projects through the wall and/or cover plate of the filter housing so that they can be adjusted from outside the filter housing. A more detailed description of how a such a filter operates can be found in the book by G. Mattaei et. al cited above.
Also, in the U.S. Pat. Nos. 5,023,579 and 4,775,847, and in the European patent application EP 0 817 301 A1, conventional radio frequency filters are disclosed.
In FIG. 1, a general view of a band-pass filter 20 according to the state of the art is shown. Generally such a filter comprises a filter housing 18, an input terminal 22 at a first end of the housing and an output terminal 24 at a second end of the housing. Inside the housing resonators 26 are located. Above the resonators tuning screws 28 are located. The tuning screws are secured by lock nuts 30.
The lock nuts 30 are provided so that the tuning screws, once they have been adjusted to the correct position, can be locked in this position. This is very important in a filter of the type described above, since only a small deviation from the correct position of one of the tuning screws may result in a malfunction of the filter.
However, the use of lock nuts is associated with a problem. Thus, once the tuning screws have been screwed to the correct position and the filter is tuned correctly, the screws should be secured in a locked position by means of the lock nuts. When the lock nuts are brought in place, it is almost impossible to not change the position of the tuning screws. The tuning screws must then be re-adjusted. This procedure must in some cases be repeated more than one time, which of course is time consuming and therefore undesired.
It is an object of the present invention to overcome the drawbacks of the securing arrangements according to the prior art and to provide a screw which is easy to secure and which does not need to be readjusted once adjusted to the correct position.
This object and others are obtained by a screw threaded through a sleeve in a radio frequency filter, the sleeve having at least one portion having a lateral dimension being smaller than the diameter of the screw and which portion is flexible in a radial direction.
Thus, when the screw is threaded through the sleeve, the portion of the sleeve having the smaller dimension will impose a clamping force on the screw. The increased friction provided by this clamping force will hold the screw in a locked position. The magnitude of the clamping force can be varied within broad ranges, and the optimum force will be different for different applications.